Various flowmeters are used to measure the flow rate of a fluid through the meter. Some examples include vortex flowmeters, magnetic flowmeters, Coriolis flowmeters, and others.
A Coriolis flowmeter is one type of flowmeter that is used to measure mass flow rate and density of a fluid. In a conventional Coriolis flowmeter, fluid flows through one or more conduits (broadly, a flowtube) as one or more drivers oscillates the conduits. Two or more sensors detect the motion of the flowtube at different locations along the flowtube. Forces associated with acceleration of fluid flowing through the oscillating flowtube produce a phase difference in the signals from the sensor. This phase difference is related to the mass flow rate of fluid through the flowtube. Thus, a measurement of mass flow rate can be obtained from the phase difference in the sensors. The resonant frequency of the flowtube varies with density of the fluid in the flowtube. Thus, a Coriolis meter can also provide a measure of density by tracking the resonant frequency of the flowtube. Although these basic principles of operation are fairly simple, many sophisticated control and measurement techniques can be used to improve the basic measurement techniques to compensate for various factors, such as temperature changes, pressure changes, effects of multiphase fluid flow, and many other variables that affect operation of the Coriolis meter. For example U.S. Pat. Nos. 6,311,136; 6,505,519; 6,950,760; 7,059,199; 7,614,312; 7,660,681; 7,617,055; and 8,751,171, the contents of which are each hereby incorporated by reference, illustrate some of the more advanced techniques known for improving measurement and operation of a Coriolis meter.
Measurement accuracy in Coriolis flowmeters depends, in part, on whether the meter is properly calibrated. Typically, calibration data is used to convert raw sensor signals to mass flow and density measurements. This calibration data can, for example, account for the physical properties of the flowtube (e.g., stiffness, etc.). In some applications, the fluid flowing through a Coriolis flowmeter is corrosive, abrasive, caustic, etc. Such fluids and/or other environmental conditions can cause a flowtube to become damaged through wear or other breakage. However, when the physical properties of the flowtube change, the calibration data becomes out of date, and the measurement system can produce inaccurate measurements. Thus, normal wear on the Coriolis meter can cause deterioration of performance. Eventually, the damage from wear can become so severe that the flowmeter catastrophically fails. Catastrophic failure of a Coriolis flowmeter can cause substantial and costly damage to the process facility where it is used, particularly where it results in release of materials that are corrosive, caustic, harmful to the environment or otherwise difficult to clean up.
To avoid catastrophic failure and limit measurement inaccuracies, some Coriolis flowmeters are equipped with meter verification systems. These systems monitor the calibration accuracy of a flowmeter to verify whether it is producing reliable measurements. They typically output a qualitative indication of whether the current measurements are accurate. For example, U.S. Pat. No. 5,926,096, the contents of which are hereby incorporated by reference, provides more detail on one type of meter verification system that can be used in a Coriolis meter. Other meter verification systems are also used in Coriolis meters. Similar meter verification systems are used in connection with vortex flowmeters, magnetic flowmeters, and other flowmeters.
The present inventors have made certain improvements in the field of flowmeter verification systems, which will be described in detail below.